The best reason to learn Greek is to be able to communicate in Greece when you are visiting there, either on business or on vacation. The Greeks love it when you show an effort and willingness to speak their language. Although many Greeks speak English to one degree or another, they like it when you use their language, even if you are not that fluent in it.
To be able to read a Greek menu and order a meal in Greek, go to a souvenir store to shop and bargain in Greek, go on a boat ride, make a reservation at a hotel, ask for directions, or tell a cab driver where you want him to take you, things like this help make your stay in Greece more pleasant. Plus, you are not so likely to be overcharged for services when you give the impression that you are on the ball.
